This invention relates to coating photoreceptor substrates using dip tanks and more particularly to coating photoreceptor substrates using a dip tank arranged with a plurality of layers of rounded objects or beads.
It is known to use coating fluid reservoirs or xe2x80x9cdiptanksxe2x80x9d to apply photoreceptor coating solution to coat photoreceptor devices such as photoreceptor flexible belts and cylindrical-shaped drums. In the foregoing U.S. Pat. No. 5,681,392 to Eugene A. Swain, for example, the fluid reservoir (equivalent to a diptank) 10 is used to supply organic photoreceptor coating fluid 80 to coat a flexible belt-type photoreceptor substrate 60.
In this coating process, a photoreceptor substrate (belt or drum) is immersed or xe2x80x9cdippedxe2x80x9d into the orifice of a tank containing the solution to be coated and then withdrawn at a rate that controls the coating thickness. The usual mechanism to coat the substrate is to pump a coating solution containing the active materials, either dissolved or in suspension (such as pigments), into the tank from an inlet located in the bottom of the diptank and continuously overflow the tank at the orifice located at the top of the tank. In this way the substrate is subjected to a uniform flow of solution relative to the coating speed.
There are several disadvantages to the dip coating process which can result in defects on the coated substrate surface.
For example, typically there is very little radial surface velocity of the coating solution at the top of the tank. In fact, usually there is a conical volume in the tank where there is relatively little coating solution flow. As a result of non-uniformities in the coating solution, coating streaks can occur along part or all of the dipped length of the photoreceptor substrate. Such non-uniformities can occur especially from dispersions that have poor stability and display a property of non-uniform dispersion distribution called flocculation.
As is known, flocculation occurs when there is little or no movement or shear of the solution, such as the conical volume of the tank discussed above. Flocculation results in solvent-rich and pigment-rich zones in the dip tank. Unfortunately, such zones are exactly where the photoreceptor substrate is immersed. Ultimately, these phenomena can result in coating streaks or other defects in the resulting finished photoreceptor device.
As a result, there is a need for an improved apparatus and method for coating photoreceptor substrates.
In one aspect of the invention, there is provided an apparatus for coating at least one substrate with a fluid. The apparatus comprises at least one diptank defining an inlet and a conduit with an orifice, the conduit including means for suspending a plurality of layers of rounded objects so that fluid supplied to the inlet flows through the plurality of layers of rounded objects to coat a substrate that is inserted through the orifice.
In another aspect of the invention, there is provided a method for coating at least one substrate with a fluid. The method uses an apparatus comprising at least one diptank defining an inlet and a conduit with an orifice, the conduit including means for suspending a plurality of layers of rounded objects, so that fluid supplied to the inlet flows through the plurality of layers of rounded objects to coat a substrate that is inserted through the orifice. The method comprises supplying fluid to the inlet and inserting at least one substrate through the orifice.